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Deep sea sharks pig out on beef, lamb and veg we throw away

Just wait, and dinner will come floating down. That seems to be the feeding strategy for sharks and other bony fish that thrive in the western Mediterranean Sea, at depths exceeding 2000 metres.

Beef, goat meat, dolphin blubber, vegetables and fruit – even a bunch of grapes, for example – were gratefully received and gobbled down by these fish, living in or around a canyon called the Valencia trench, midway between Barcelona, Spain, and the Balearic Islands.

Joan Cartes of the Institute of Marine Sciences in Barcelona and his team unpicked deep-sea dining habits by catching 445 live fish – mainly three species of shark and six bottom-dwelling bony fish – and analysing their gut contents.

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The work is the first to assess the extent to which deep-dwelling fish rely on bounty from above. It was also the first to examine how much of what they ingested was derived from human activity – food jettisoned from ships, plastic rubbish, debris from fishing nets and so on – versus what could be said to be sourced from nature.

The researchers say our food waste and debris could be affecting deep-sea food webs, which are already under stress from trawling.

The fishes’ guts included microplastic fragments and strands from fishing nets, although in amounts too small to cause problems for large fish. “Plastics were frequently found, but in small volumes of the total diet, typically less than 2 per cent by weight,” says Cartes.

Much more important were foodfalls of other dead fish and animals, which provided as much as 70 per cent of the stomach contents in some fish, particularly sea sharks such as Portuguese dogfish and blackmouth catfish. The remains of members of their own species accounted for much of what they ate.

A surprise, however, was the range of food surviving at that depth from human activity, which accounted for around 5 to 6 per cent of the diet. Such items turned up in fish living beneath routes frequently plied by ferries, and within fishing grounds.

Cartes thinks that the same may be happening elsewhere in the world, and so could be an underappreciated factor affecting deep-sea ecology. “I think all deep-sea systems in the world may be exposed to similar impacts,” he says.

“Humans have probably had all kinds of effects on food availability to deep-sea scavengers through fisheries discards, changes in fish stocks and whaling,” says David Bailey of the University of Glasgow, UK. “In the enclosed Mediterranean, the effects of food from the land are likely much stronger than out in oceans. So while I’m not surprised by these findings, they’re interesting and show how much we’re affecting deep-sea systems, even though they’re out of sight.”